The present invention relates to techniques for alignment of CRT based display systems which permit all major CRT-related alignment to be performed at a factory or other centralized facility, with automatic precision alignment in the field of the overall display system including a CRT assembly as one element thereof. The invention particularly facilitates field replacement of a CRT assembly, while avoiding any requirement for a difficult field alignment procedure. While the invention is applicable to any CRT based display system, it is applicable in particular to a high resolution cathodochromic CRT projection display.
At this point, it may be noted that an image target on which an electron beam impinges in a cathodochromic CRT does not emit light as does an image target in a cathodoluminescent CRT. Rather, the cathodochromic materials employed change color when excited by an electron beam. In the case of an image target comprising cathodochromic bromine sodalite, the resultant coloration remains indefinitely, until deliberately erased. In addition to inherent memory, cathodochromic image targets have the properties of high resolution, and high contrast in bright ambient light making them highly suitable for projection systems.
Erasure of a cathodochromic image target is normally effected by heating to about 300.degree. C. An economical and technically feasible erasure method is electron beam heating, wherein the image target is scanned, in a raster pattern, with an electron beam spot energy density such that temperature is raised above an erase threshold. While highly effective, electron beam erasure is a relatively critical operation in that the electron beam current and focus must be such as to achieve a sufficient energy for erasure, without reaching a damage threshold which causes permanent damage to the image target.
Processes for preparing cathodochromic sodalite and a cathodochromic CRT projection display are disclosed in Todd, Jr. et al U.S. Pat. No. 3,932,592 and Todd, Jr. U.S. Pat. No. 3,959,584, to which reference may be had for further details.
As is well known, replacement of a CRT in any display system normally requires an electronic alignment procedure in view of sample variations from one CRT to the next, even among CRTs of the same model or type number. This alignment problem is present, in one degree or another, in virtually all CRT based display systems, ranging from simple monochrome television sets through high resolution video data displays, including color video displays, as well as in projection CRT display systems, including cathodochromic CRT projection displays such as are disclosed in the aboveidentified Todd, Jr. U.S. Pat. No. 3,959,584. Alignment is generally a relatively time consuming iterative procedure normally involving a number of interdependent adjustments.
There are three major areas of electronic alignment involved in field replacement of any monochrome or cathodochromic CRT, namely: (1) electron gun drive (beam current) adjustment; (2) beam focus alignment; and (3) deflection/geometry correction. Color CRT systems in addition to these three require convergence alignment.
Of the three major areas identified above, electron gun drive (beam current) and beam focus are particularly critical ones in relatively higher power systems where energy imparted by an electron beam impinging on an image target within the CRT is close to the level which will cause permanent damage. Beam current and spot size (focus) together determine the energy density of the electron beam spot where the electron beam impinges on the image target. Assuming a simple raster scan pattern, scan speed and overlap from one line to the next will also affect heating. By way of example, assuming a 30 kV fixed anode voltage for accelerating the electron beam, for erasing a cathodochromic CRT image target the CRT is operated such that spot diameter is 10 mils and electron beam current is 500 microamperes. In contrast, for normal image writing, a typical spot size is 1.5 mils with an electron beam current of 50 microamperes.
Two particular situations where energy density of the electron beam spot is important are electron beam erasure of cathodochromic CRT projection tubes, and operation of high brightness cathodeoluminescent projection CRTs in general. For erasing a cathodochromic CRT, the CRT is typically operated such that the energy density of the electron beam spot is near the maximum allowable, but still below a threshold level at which damage results. The damage to be avoided is primarily an overheating effect, and is typically manifested by effects such as release of binder material holding active components of an image screen composition together, fusing of the image screen composition, damage to the chemical structure of the phosphor, or even faceplace cracking in some systems.
Proper electron beam current must of course be maintained at all times during operation. However, particular care must be taken during the alignment and adjustment process itself. Damage is most likely to occur at this point, since the adjustments are being manipulated. Manifestly, a high degree of skill is required to ensure that electron beam current does not reach the threshold level where damage results.
Briefly considering the second alignment area identified above, beam focus, in CRTs used in high resolution applications there is usually a provision for both static (center screen) focus and dynamic focus to maintain edge focus as close as possible to the center screen focus. Static focus is a simple adjustment in a monochrome CRT, and dynamic focus involves generally three or four interdependent adjustments that generate a signal which is summed either electrically or magnetically with the static focus level signal.
Briefly considering the third alignment area, deflection/geometry correction, due to the geometries involved in electron beam scanning of the CRT image area, a number of correction factors must be added to the basic horizontal and vertical deflection signals. The number of adjustments per axis ranges from a minimum of three to as many as twelve in high resolution projection applications. Geometry correction is significantly more complex for a projection display system compared to a direct-view CRT system because a projection optical system normally introduces distortions, such as trapezoidal and linearity distortion as a result of projection angle and lens design considerations, and these must be compensated for to achieve an aligned display as actually presented to a viewer.
Moreover, a cathodochromic CRT generally has multiple focus levels to accommodate different operating conditions such as writing and erase, while a cathodeoluminescent CRT normally has just one. Deflection and geometry correction correspondingly must change for different operating conditions of a cathodochromic CRT. For example, for writing in a raster-scan system, a well-focused spot scanning over a portion of the image target is required. This portion of the image target will generally be trapezoidal rather than rectangular to compensate for the optical geometry so that the ultimate projected image is rectangular. However, for electron beam erasure, the parameters for deflection/geometry correction should be set to achieve a uniform energy density over the entire image target, even outside the normal visible areas. Again, proper alignment is a critical and complex procedure, not easily done in the field.
Particularly in the context of the present invention, it is significant to note that the alignment requirements briefly discussed above can be further characterized as having two distinct sources: (1) display system design, including the geometry of projection optical systems; and (2) manufacturing or sample variations in both the electronics and the CRT itself from one particular unit to the next. Heretofore, manufacturing variations at least have necessitated that manual adjustments be provided to compensate, at field installation, for such variations. In more demanding applications, such as projection, high resolution, and combinations of both, it will be appreciated that the adjustment procedure becomes relatively sophisticated and complicated.
Evidencing the importance of proper alignment, particularly in high-resolution display systems, there have been a number of proposals directed to control and correction of CRT-based display systems, often employing digital techniques. For example, Paul C. Lyon, in "A Wide Field-of-View CRT Projection System with Optical Feedback for Self Alignment", Evans & Sutherland Computer Corporation, describes a multi-channel color CRT projection system including a microprocessor-based subsystem involving optical feedback to provide self-alignment for color-hue, intensity, color-convergence, geometry, and focus. The article describes, by way of background, one general technique for geometry correction, namely, analog function generator circuitry to generate a correction polynomial as a function of X and Y screen position coordinates, with adjustable coefficients to adjust the weighting of each product term in the correction polynomial. In the actual implementation described in the Lyon article, a digital correction memory is used instead of analog function generator, and the various product terms of the correction polynomial are effectively locked together in the digital correction memory after being calculated and stored there by the microprocessor operating in an optical feedback mode.
Other CRT display systems which calculate correction polynomials based on stored constants, but which lack optical feedback for automatic correction as in the Lyon system, are disclosed in Judd U.S. Pat. No. 4,354,143 and Wrona U.S. Pat. No. 4,441,057.
As another example, Chase et al U.S. Pat. No. 4,385,259 discloses a dynamic convergence compensation system for a shadow mask color CRT display wherein "coarse" compensation is provided by analog circuitry which generates primary terms of a polynomial and "fine" compensation is provided by digital PROMs, in which are stored representations of the coefficients of remaining terms of the polynomial. While Chase et al describe several different adjustment techniques, of particular note in the context of the present invention is one where a CRT and its associated deflection coils are supplied "as a precalibrated unitary assembly from a manufacturer." The manufacturer also supplies a set of recommended convergence waveforms he has established for the particular unit. Based on a study of these curves, correction currents are determined and employed to adjust the values of certain resistors in the analog circuitry and to determine the contents of the PROMs.
Yet another example of circuitry for digital control and correction of signals used to drive a CRT is disclosed in Hallett et al U.S. Pat. No. 4,203,051. In this particular system, the electronics for generating the deflection waveforms for a color CRT includes a pair of memories. The first memory is a basic waveform store from which is derived the basic waveform which controls the convergence of all three color beams. The other memory is an error correction store from which are derived correction currents for finer adjustment. It appears that the basic waveforms are determined "for each CRT during manufacture", while the data for the error correction memory are determined during a field adjustment procedure.
In Bristow U.S. Pat. No. 4,099,092 a display alignment technique is described where a centralized test station is used for precision alignment of a display system as a whole, and the results are stored in a PROM. During later operation in actual use, data are read from the PROM to generate correction signals. A similar technique is disclosed in Kamata et al U.S. Pat. No. 4,401,922, wherein circuitry determines correction values for a display system, and stores the correction values in a PROM.
These and various other prior art approaches in general do not address the problems of field-replacement of components in a high resolution display system, particularly projection display systems including cathodochromic CRTs. As noted above, there are two distinct sources of alignment requirements, display system design and manufacturing or sample variations, and field adjustments are normally required following component replacement. Adjustment of a cathodochromic CRT projection system requires particular sophistication, due both to the very real possibility of damage if the electron gun drive is improperly adjusted, and to the fact that it is difficult to observe the effects of adjustment changes on a test pattern because an image, once written, remains until it is erased. Thus, it is difficult to determine which lines of a test pattern are currently being written and which are left from previous adjustments Exacerbating this difficulty, a cathodochromic CRT cannot generally be erased until a basic alignment has been done such that the electron beam can be properly directed and controlled for erase. Thus, again, alignment requires sophistication and equipment not generally available in the field. As a further complication, alignment is different for various modes of operation, and thus may need to be changed even while the system is operating.